1. Field of the Invention
The method of the present invention pertains to hindered settling of proppant in vertical fractures by the addition of low density material into the fracturing slurry comprising of fracturing fluid and proppant.
2. General Background of the Invention
Hydraulic fracturing involves the creation of a crack in a reservoir to enhance hydrocarbon production. Highly viscous fluids blended with proppant or sand is injected into the crack. When the fracture closes, the proppant left in the fracture creates a large flow area and a highly conductive pathway for hydrocarbons to flow into the wellbore. The proppant or sand is utilized to maintain an open fracture. The viscous fluids are utilized to transport, suspend and eventually allow the proppant to be trapped inside the fracture. the fluids typically follow power law behavior for the range or shear rates encountered in hydraulic fracturing treatments.
Typical fluid volumes in a propped fracturing treatment can range from a few thousand to a few million gallons. Proppant volumes can approach several thousand cubic feet. The goal is to obtain uniform proppant distribution; and therefore, a uniformly conductive fracture along the wellbore height and fracture half-length. However, the complicated nature of proppant settling in non-Newtonian fluids often causes a higher concentration of proppant to settle down in the lower part of the fracture. This often leads to a lack of adequate proppant coverage on the upper portion of the fracture and the wellbore. Clustering of proppant, encapsulation, bridging and embedment are few phenomena that lower the potential conductivity of the proppant pack.
Proppant transport inside a hydraulic fracture has two components when the fracture is being generated. The horizontal component is dictated by the fluid velocity and associated streamlines which help carry proppant to the tip of the fracture. The vertical component is dictated by the terminal particle settling velocity of the particle and is a function of proppant diameter and density as well as fluid viscosity and density. The terminal settling velocity is further complicated by the various phenomena mentioned earlier.
At some point of the fracture generation process, proppant coverage reaches an equilibrium geometry, above which all the proppant injected is carried out farther into the fracture. This potentially restricts the effective propped fracture height as well as greatly increases potential for bridging during treatment.
When pumping ceases, the fracture eventually closes on the proppant pack. The fluid contains viscosity breakers that lower the apparent viscosity and aid the acceleration of fracture closure due to faster leakoff. This reduction in fluid viscosity of the static fluid leads to higher settling velocities causing more proppant to drop to the fracture bottom. The potential for voids and non-uniform packing contribute to a lower effective propped area and hence significantly lower effective wellbore radius as closure occurs.
Other inventions aimed at prevent proppant settling in a vertical fracture have focused on creating proppant with density equal to that of the carrier fluid. Thus, the proppant in the fluid would be neutrally buoyant in the fracture and stay in place vertically into the fracture closure. The methods of creating neutrally buoyant proppant includes surface-sealing of porous ceramic particles to trap air-filled voids inside the particles, creating composites of strong materials and hollow ceramic spheres, and creating hollow spheres with sufficient wall strength to withstand closure stresses.
These approaches all possess inherent drawbacks in terms of proppant durability and cost to manufacture.